1. Field of the Invention
The present invention relates to organic electroluminescent devices, and more particularly, to a top emission type organic electroluminescent device.
2. Discussion of the Related Art
Flat panel display devices—which are characterized as being thin, light weight and energy efficient—are in high demand in the display field as the information age rapidly evolves. Flat panel display devices may be classified into two types depending on whether it emits or receives light. One type is a light-emitting type display device that emits light to display images, and the other type is a light-receiving type display device that uses an external light source to display images. Plasma display panels, field emission display devices, and electroluminescence display devices are examples of the light-emitting type display devices. Liquid crystal displays are examples of the light-receiving type display device.
Among the flat panel display devices, liquid crystal display (LCD) devices are widely used for laptop computers and desktop monitors because of their superiority in resolution, color image display and image quality. However, since the LCD device is the light-receiving type display device, it has some disadvantages such as poor contrast ratio, narrow viewing angle, and difficulty in enlarging its size. Therefore, new types of flat panel display need to be researched and developed to overcome the aforementioned disadvantages.
Recently, organic electroluminescent display devices have been of the most interest in research and development because they are light-emitting type displays having a wide viewing angle and a good contrast ratio as compared to the LCD device. Since the organic electroluminescent device is a light-emitting type display device, it does not require a backlight device and can be light weight and thin. Further, the organic electroluminescent display device has low power consumption. When driving the organic electroluminescent display device, a low voltage direct current can be used, and a rapid response speed can be obtained. As widely known, since the organic electroluminescent display device is totally solid phase unlike the LCD device, it is sufficiently strong to withstand external impacts and has a greater temperature range. Additionally, the organic electroluminescent device can be manufactured at a low cost. Moreover, since only the deposition and encapsulation apparatuses are necessary in a process of manufacturing the organic electroluminescent display device, process management can be simplified, and suitable procedures can be achieved.
As an operating method for the organic electroluminescent display device, a passive matrix operating method not using thin film transistors is conventionally utilized. In this type of organic electroluminescent display device, scanning lines and signal lines, which are arranged in a matrix pattern, perpendicularly cross each other. The scanning voltage is sequentially applied to the scanning lines to operate each pixel. To obtain the required average luminance, the instantaneous luminance of each pixel during the selection periods is intensified by multiplying the average luminance by the number of scanning line.
However, as another method of operating the organic electroluminescent display device, an active matrix operating method that is free from the above-described problems is proposed. The active matrix type organic electroluminescent display device usually includes thin film transistors pairs which confer a voltage storing capability on the pixels. Each of the pairs of thin film transistors has a selection transistor and a drive transistor. The selection transistor is connected to a signal line for supplying a data signal and a scanning line for supplying a gate signal. The drive transistor is connected to the selection transistor and a constant voltage line. In the structure of active matrix type organic electroluminescent display device, a voltage applied to the pixels is stored in storage capacitors, thereby maintaining the signals until the next period of applying the voltage. As a result, a substantially constant current flows through the pixels, and the organic electroluminescent display device emits light at a substantially constant luminance during one frame period. With the active matrix type, because a low current is applied to the pixel, it is possible to enlarge the display device, thereby forming much finer patterns and obtaining a much lower power consumption due to the constant luminance.
The driving principle of the display apparatus according to the conventional art will now be described. FIG. 1 is an equivalent circuit diagram which specifically shows the structural basis of pixels of the active matrix type organic electroluminescent display device.
As shown in FIG. 1, scanning lines are arranged in a transverse direction, and signal lines are arranged in a longitudinal direction perpendicular to the scanning lines. A voltage line that is connected to a power supply to provide a voltage to drive transistors is also disposed in the transverse direction. A pair of signal lines and a pair of scanning lines define a pixel area. Each selection transistor (i.e., commonly called switching thin film transistor (TFT)) is disposed in the pixel area near the crossing of the scanning line and signal line and acts as an addressing element that controls the voltage. A storage capacitor CST is connected to the voltage line and the switching TFT. Each drive transistor (i.e., commonly called driving TFT) is connected to the storage capacitor CST and the voltage line and acts as a current source element. An organic electroluminescent diode is connected to the drive transistor.
The organic electroluminescent diode had a double-layer structure of organic thin films between an anode electrode and a cathode electrode. The organic thin films and the fabrication technologies have been improved. As a result, organic electroluminescent diodes presently available provide colors in the emitted light. Since the primary three colors have been obtained, research and development have been directed toward providing a full-color organic electroluminescent element.
When the forward current is applied to the organic electroluminescent diode, the electron-hole pair is combined through the P(positive)-N(negative) junction between the anode electrode providing the hole and the cathode electrode providing the electron. The electron-hole pair has a lower energy than when they are separated into the electron and the hole. Therefore, an energy gap occurs between the combination and the separation of electron-hole pairs, and this energy is converted into light by the organic electroluminescent element. That is, the organic electroluminescent layer absorbs the energy generated due to the recombination of electrons and holes when a current flows.
The organic electroluminescent devices are classified into a top emission type and a bottom emission type in accordance with a progressive direction of light emitted from the organic electroluminescent diode. In the bottom emission type device, light is emitted in a direction toward the substrate where the lines and TFTs are disposed. Therefore, the display area decreases because the emitted light is blocked by the lines and TFTs. However, in the top emission type device, since light is emitted in a direction opposite to the substrate, the display area can be 70–80% of the entire panel area.
The top emission type can have a low contrast ratio as compared to the bottom emission type because of the effects of external light reflection. The contrast ratio in the organic electroluminescent device is a luminance ratio when the device is turned on and off. The luminance during the time the device is turned off is determined by the device's reflection ratio to the external light. Therefore, it is important to decrease the device's reflection ratio to external light in order to obtain a high contrast ratio.
FIG. 2 is a partial cross-sectional view showing an example of a top emission type organic electroluminescent display device according to a conventional art. In FIG. 2, an organic electroluminescent display device includes a driving thin film transistor (TFT) T and an organic electroluminescent diode E. A buffer layer 30 is formed on a substrate 1. The driving TFT T includes a semiconductor layer 32 on the buffer layer 30, a gate electrode 38, a source electrode 50 and a drain electrode 52. A power electrode 42 extending from the voltage line is connected to the source electrode 50 and the organic electroluminescent diode E is connected to the drain electrode 52. A capacitor electrode 34 made of the same material as the semiconductor layer 32 is disposed below the power electrode 42. The power electrode 42 corresponds to the capacitor electrode 34 and an insulator is interposed therebetween, thereby forming a storage capacitor CST.
The organic electroluminescent diode E includes an anode electrode 58, a cathode electrode 66 and an organic electroluminescent layer 64 interposed therebetween. The organic electroluminescent device shown in FIG. 2 has a luminous area A where the organic electroluminescent diode E emits light produced therein.
A top passivation layer 68 is formed on the cathode electrode 66 to protect the organic electroluminescent elements from external environmental effects, such as humidity. As a material for the top passivation layer 68, an organic or inorganic material can be used. However, since the organic or inorganic material has a refractive index of more than 1.5, a surface reflectivity of about 4% may result. Therefore, the contrast of the organic electroluminescent device is degraded. Furthermore, the anode electrode 58 of the organic electroluminescent diode E may be made of Au, Ag, Pt, Al or the like which has a high reflectivity, for example, more than about 60%. Thus, the contrast ratio due to external environmental effects is dramatically degraded because of the reflection on the anode electrode 58 made of one of those metallic materials.
Although not shown in FIG. 2, a circularly polarizing plate which controls a phase difference of incident light can be formed on the top passivation layer 68. However, since the circularly polarizing plate is degraded by humidity and high temperature, the lifetime of the products becomes shorter and the product costs increases.
FIG. 3 is a partial cross-sectional view showing another example of a top emission type organic electroluminescent display device according to the conventional art.
As shown in FIG. 3, the organic electroluminescent display device includes a thin film transistor (TFT) T within a luminous area A. The thin film transistor T includes a gate electrode 12, a semiconductor layer 16, and source and drain electrode 18 and 20. An organic electroluminescent diode E connected to the thin film transistor T has a lower electrode 24, an upper electrode 28 and an organic electroluminescent layer 26 interposed therebetween. Here, an insulation layer 27 divides the organic electroluminescent layer 26 into a pixel unit, and the organic electroluminescent layer 26 overlaps the thin film transistor T.
The lower and upper electrodes 24 and 28 of the organic electroluminescent diode E become a cathode and/or an anode, respectively, depending on a carrier type supplied from the thin film transistor T. When an n-type TFT having electrons as carriers is connected to the organic electroluminescent diode E, the lower electrode 24 becomes the cathode and the upper electrode 30 becomes the anode. However, when the p-type TFT having holes as carriers is connected to the organic electroluminescent diode E, the lower electrode 24 becomes the anode and the upper electrode 30 becomes the cathode.
Still in FIG. 3, a buffer layer 29 is formed on the upper electrode 28 and a passivation layer 30 is formed on the buffer layer 29. The buffer layer 29 is an insulation material that can be deposited on the upper electrode 28 by a vacuum evaporation method. Additionally, the buffer layer 29 protects the organic electroluminescent diode E when the passivation layer 30 is formed. As a material for the passivation layer 30, a thick insulation material or a glass plate is usually employed.
However, the organic electroluminescent display device shown in FIG. 3 has a number of problems. For example, because the lower electrode is usually formed of Au, Ag, Pt, Al or the like which has a reflectivity of more than 60%, external light is easily reflected by the lower electrode in high luminance intensity situations. As a result, such a reflection causes reduction in contrast.